My guess would be that in developed countries, people are having kids older and older, so the grandparents are accordingly also older and more tired. That combined with multi-generational households being all but gone so now you're picking up and leaving off and all the kind of cooking and general housekeeping is also doubled.
If your parents had you at 25 and you have kids at 25 then your parents are 50 when you have a kid. Nowhere close to retirement age. People who are still working can't watch the kids five days a week.
FWIW the average age of first-time mothers in San Francisco is 33.6. It seems that still speaks against my theory. Maybe it's more that previous generations of women were less likely to have full-time employment at all?
there is also the sentiment of people in their 50s that they are done with taking care of kids. they want to enjoy their freedom now. it's just an anecdote, but for example my dad tried to marry again, but was unable to find a partner willing to marry someone with kids. i don't know if that translates to taking care of grand kids, but i think it is related.
in developing countries there are no pensions for many people and the young parents provide the support the grandparents need, taking care of the grandkids is one way to ensure that this support keeps coming.
